It is known, as the aforedescribed applications indicate, to automatically monitor an environmental condition, such as the concentration of methane in a ventilating air current for a mine by analyzing the air for the particular component of concern and comparing the measured values with a normal or desired state to establish an alert condition when and if the level of the component exceeds the predetermined maximum.
In general, moreover, it is known to measure magnitudes of the concentrations of the components of the atmosphere for a variety of applications to obtain long term information regarding the atmosphere or for short term purposes, e.g. to determine whether levels of the various components exceed or fall below desired magnitudes. These measurements are generally done directly utilizing various processes and techniques which have been found to be effective for the different components.
Probably the most significant of these measuring stations are those which are located in environments with potential danger to operating personnel, i.e. to signal the reliability of ventilation, to indicate a condition of potential danger requiring evacuation of personnel or to trigger remedial measurements (e.g. increased ventilation) to eliminate dangerous conditions. Such stations are located in mines and other subterranean facilities.
For the evaluation of the concentrations of the several components, as noted, a variety of measuring systems have been and are used in practice. For example, it is known to measure the concentration of explosive and combustible gases, especially fire damp, hydrogen and most specifically methane, by a system which utilizes a catalytic combustion of the component. This system will be referred to as the "catalytic combustion" system hereinafter. For the measurement of the concentrations of toxic gases, it is known to determine the rate at which such toxic components are chemisorbed on metal oxide semiconductors, this system being referred to hereinafter as the "Chemisorption on Metal Oxide Semiconductor" system. The measurement of the concentration of oxygen can be effected by a process in which the oxygen traverses a membrane and a cathode juxtaposed with an anode across a basic electrolyte, an electric current being thus generated to represent an oxygen concentration. The latter system will be described as the "Chemical Current-Generating System". For the quantitave determination of carbon dioxide, it is common to utilize a measurement in the change of thermal conductivity and hence this system can be referred to as the "Thermal Conductivity" system.
Reference may be made to the "VABOTECTOR-EX" instrument marketed by Firma Gesellschaft fur Geratebau mbH & Co. KG, of the Federal Republic of Germany as described in a brochure 6/82 thereof. Naturally, other parameters of the atmosphere can be measured, these including pressure, temperature, relative humidity and the like by appropriate measuring systems (see the aforementioned applications).
Indeed, even for the measurement of one parameter, it is possible that a number of measuring systems will be required for various reasons and to suit various boundary conditions. This is particularly the case for the measurement of methane concentration of a gas. For high precision at low concentrations, for example, it is known to utilize the catalytic combustion system while for higher concentrations, it may be necessary to utilize the thermal conductivity system. In other words, under certain circumstances where one system may not be available or cannot effectively be used, it may be essential to utilize a second system. A case in point is where the instrument is a battery-powered portable instrument and one system may require a higher energy consumption than another and low battery power may make the use of the high-consumption system ineffective.
As described in my commonly owned German patent application No. P 32 43 542.8-52 (German patent document No. DE-OS 32 43 542), which was published May 30, 1984, a catalytic combustion system is utilized and in the event of a problem, the system can switch over to the thermal conductivity system for measuring methane concentration. The system is utilized to determine approach to a lower explosion limit.
While this system has been found to be highly effective, there is a general problem in this field and that is a problem of sensitive or dangerous instruments or measuring techniques at least for some parameters such as the concentrations of certain components of the gas stream. It is the case with methane for example, and in the case of methane or wherever a sensitive measuring system which is not completely reliable has been used in the past, it is desirable to find a less sensitive or dangerous system for measuring the concentration of the particular component.